Publications

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2015
Wei, SS, Wiens DA, Zha Y, Plank T, Webb SC, Blackman DK, Dunn RA, Conder JA.  2015.  Seismic evidence of effects of water on melt transport in the Lau back-arc mantle. Nature. 518   10.1038/nature14113   AbstractWebsite

Processes of melt generation and transport beneath back-arc spreading centres are controlled by two endmember mechanisms: decompression melting similar to that at mid-ocean ridges and flux melting resembling that beneath arcs'. The Lau Basin, with an abundance of spreading ridges at different distances from the subduction zone, provides an opportunity to distinguish the effects of these two different melting processes on magma production and crust formation. Here we present constraints on the three-dimensional distribution of partial melt inferred from seismic velocities obtained from Rayleigh wave tomography using land and ocean-bottom seismographs. Low seismic velocities beneath the Central Lau Spreading Centre and the northern Eastern Lau Spreading Centre extend deeper and westwards into the back-arc, suggesting that these spreading centres are fed by melting along upwelling zones from the west, and helping to explain geochemical differences with the Valu Fa Ridge to the south(2), which has no distinct deep low-seismic-velocity anomalies. A region of low S-wave velocity, interpreted as resulting from high melt content, is imaged in the mantle wedge beneath the Central Lau Spreading Centre and the northeastern Lau Basin, even where no active spreading centre currently exists. This low-seismic-velocity anomaly becomes weaker with distance southward along the Eastern Lau Spreading Centre and the Valu Fa Ridge, in contrast to the inferred increase in magmatic productivity(1). We propose that the anomaly variations result from changes in the efficiency of melt extraction, with the decrease in melt to the south correlating with increased fractional melting and higher water content in the magma. Water released from the slab may greatly reduce the melt viscosity(3) or increase grain size(4), or both, thereby facilitating melt transport.

2007
van Wijk, JW, Blackman DK.  2007.  Development of en echelon magmatic segments along oblique spreading ridges. Geology. 35:599-602.   10.1130/g23294a.1   AbstractWebsite

En echelon magmatic segments commonly develop along obliquely spreading oceanic ridges. To clarify some of the dynamic aspects of this plate boundary, we performed a series of thermo-mechanical numerical tests. When extension of oceanic lithosphere becomes oblique, deformation within the axial region localizes into distinct upwelling centers. Temperatures are elevated in the upwelling cells, which are shallow mantle features that form the new plate boundary. The predicted features are similar to the axial volcanic ridges documented at Mohns and Reykjanes Ridges, and we conclude that they become the new loci of extensional deformation, upwelling, and magmatic activity. These ridges, suborthogonal to the plate spreading direction, only develop when the axis rift zone is weak. The subsegment length and spacing depend primarily on obliquity and axial width. Predicted crustal thickness along the subsegmented axis varies discernibly; this might explain the morphology and satellite gravity of the flanks of oblique spreading ridges.

2006
Becker, TW, Schulte-Pelkum V, Blackman DK, Kellogg JB, O'Connell RJ.  2006.  Mantle flow under the western United States from shear wave splitting. Earth and Planetary Science Letters. 247:235-251.   10.1016/j.epsl.2006.05.010   AbstractWebsite

We show that SKS splitting in the westernmost United States (polarization of the fastest shear waves and splitting times, including their back-azimuthal dependence) can be explained by a geodynamic model that includes a continuum-mechanics description of plate motions and underlying asthenospheric circulation. Models that include a counterflow at depths of similar to 300 km are preferred, which may indicate a far-field effect of the Farallon slab anomaly sinking underneath the central continental United States. This finding is broadly consistent with earlier suggestions, and we demonstrate that a mechanically coupled system, though with a strong viscosity contrast with depth, is consistent with the data. We explore the depth dependence of predicted anisotropy by means of computing seismogram synthetics and comparing synthetic splits with observations. Some patterns in the data, including null observations, are matched well. Linked models of geodynamic flow and mineral alignment in the mantle provide a means to test the relationship between strain and the saturation of texturing. Lower fabric saturation strains than for global models are preferred by the data, which may reflect the relatively active tectonic setting and thin asthenosphere of the study region. In general, our results show that seismic anisotropy, when interpreted jointly with mineral physics theories, may be used to quantitatively constrain the spatial character of flow, and the degree of force coupling, at depth. (c) 2006 Elsevier B.V. All rights reserved.

2000
Hall, CE, Fischer KM, Parmentier EM, Blackman DK.  2000.  The influence of plate motions on three-dimensional back arc mantle flow and shear wave. Journal of Geophysical Research-Solid Earth. 105:28009-28033.   10.1029/2000jb900297   AbstractWebsite

Both the polarization direction of the fast shear waves and the types of deformation within overriding plates vary between the back are basins of western Pacific subduction zones. The goal of this study is to test the possibility that motions of the overriding plates may control the patterns of seismic anisotropy and therefore the observed shear wave splitting. We calculated three-dimensional models of viscous asthenospheric flow driven by the motions of the subducting slab and overriding plates. Shear wave splitting was calculated for polymineralic anisotropy within the back are mantle wedge assuming that the anisotropy was created by flow-induced strain. Predicted splitting may differ substantially depending on whether anisotropy is computed directly using polycrystalline plasticity models or is based on the orientation of finite strain. A parameter study shows that: both finite strain and textural anisotropy developed within three-dimensional, plate-coupled asthenospheric flow models are very heterogeneous when back are shearing occurs within the overriding plate. Therefore predicted shear wave splitting varies strongly as a function of plate motion boundary conditions and with ray path through the back are asthenosphere. Flow models incorporating plate motion boundary conditions for the Tonga, southern Kuril, and eastern Aleutian subduction zones produce splitting parameters consistent with observations from each region. Trench-parallel flow generated by small variations in the dip of the subducting plate may be important in explaining observed fast directions of anisotropy sampled within the innermost corner of the mantle wedge.